1. Field of the Invention
The present invention relates to for example a switch changing controlling method in a front-end circuit of a tuner that divides a wide-band television broadcast signal into a plurality of frequency bands and which can perform processing for each divided frequency band by changing and selecting a frequency band to be processed by one or a plurality of switches, and a signal processing device and a front-end circuit to which the switch changing controlling method is applied.
2. Description of the Related Art
The front-end circuit of a tuner for receiving television broadcasts can receive and process television broadcasts in various countries irrespective of broadcasting formats of the television broadcasts. However, when television broadcasts in various countries are received and used, the reception frequency band of the television broadcasts is very wide. Thus, it is generally difficult to select a received signal by one band-pass filter.
Accordingly, a front-end circuit is provided which changes a reception band according to a channel selecting operation of a user with frequencies used by television broadcasts in various countries divided into three bands as follows, for example.
(A) 46 to 147 MHz (VL band)
(B) 147 to 401 MHz (VH band)
(C) 401 to 887 MHz (U band)
FIG. 13 is a diagram showing an example of configuration of a part of a television broadcast receiver including an example of configuration of an input stage of the front-end circuit of a television tuner when the frequency band is thus divided into three bands. The front-end circuit 1 enclosed by a dotted line in the example of FIG. 13 is integrated into a one-chip IC (Integrated Circuit).
In FIG. 13, a television broadcast signal Vi received by an antenna is input to the input terminal 10 of the front-end circuit 1. The television broadcast signal Vi is supplied to band-pass filters 12A, 12B, and 12C for the three bands described above via three respective signal switches 11A, 11B, and 11C, which are turned on and off in correspondence with each of the above-described three bands. In this case, the band-pass filters 12A, 12B, and 12C have the respective frequency bands of the VL band, the VH band, and the U band described above as selected frequency bands.
A high-frequency switch using a semiconductor is used as the signal switches 11A, 11B, and 11C. High-frequency switches of this kind have recently been used with a very high frequency, and are also disclosed in Japanese Patent Laid-Open No. Hei 9-139601 (hereinafter referred to as Patent Document 1) and Japanese Patent Laid-Open No. Hei 10-284901 (hereinafter referred to as Patent Document 2), for example.
The output signals of the respective band-pass filters 12A, 12B, and 12C are supplied to respective mixers 14A, 14B, and 14C via respective low-noise amplifiers 13A, 13B, and 13C. The mixers 14A, 14B, and 14C are each supplied with a local oscillating signal. The mixers 14A, 14B, and 14C thereby frequency-convert the output signals of the respective low-noise amplifiers 13A, 13B, and 13C into lower intermediate frequencies. Then, the intermediate frequency signals from the mixers 14A, 14B, and 14C are supplied to a low-frequency processing section, so that television signals are demodulated.
In the example of FIG. 13, for example a system from the switch circuit 11A to the band-pass filter 12A to the low-noise amplifier 13A to the mixer 14A is for the VL band. A system from the switch circuit 11B to the band-pass filter 12B to the low-noise amplifier 13B to the mixer 14B is for the VH band. A system from the switch circuit 11C to the band-pass filter 12C to the low-noise amplifier 13C to the mixer 14C is for the U band.
Band changing signals SW1, SW2, and SW3 from a controlling section 2 formed by a microcomputer, for example, are supplied to the respective signal switches 11A, 11B, and 11C via respective terminal pins 15, 16, and 17.
The controlling section 2 is connected with a remote control receiving section 3. When receiving a remote control signal from a remote control transmitting section 4, the remote control receiving section 3 supplies the received signal to the controlling section 2. The controlling section 2 analyzes the received remote control signal, and performs a controlling process according to a result of the analysis.
In this case, when the remote control signal from the remote control transmitting section 4 is a channel selecting signal based on a channel changing operation by a user, the controlling section 2 first determines whether a channel indicated by the channel selecting signal is included in the VL band, the VH band, or the U band.
When the determined band is a band selected now, the controlling section 2 does not change the band changing signals SW1, SW2, and SW3 supplied to the switches 11A, 11B, and 11C. The controlling section 2 then changes and controls only the local oscillating signal supplied to the mixer in the system for the selected band so as to select the channel selected by the user.
When the determined band is different from the band selected now, the controlling section 2 selects and changes the switches 11A, 11B, and 11C by the band changing signals SW1, SW2, and SW3 so as to attain a state in which the band of the determination result is selected. The controlling section then changes and controls the local oscillating signal supplied to the mixer in the system for the band after the selection and change so as to select the channel selected by the user.
In this case, at the time of the channel change, a delay such as the pull-in time of a PLL circuit for generating the local oscillating signal and the like from the time of the change to a time at which a target broadcast channel is correctly received occurs.
The controlling section 2 therefore stops operation of a demodulating section in a stage succeeding the front-end circuit or mutes a signal supplied to the demodulating section for the period of the delay time from the time of a band change and the time of a channel change. Thereby a disturbance or the like of a reproduced image is prevented.